A variety of manufacturing processes require that successive batches of a product be temporarily immersed in a liquid chemical. In the semiconductor industry, for example, operations such as etching and cleaning of integrated circuit wafers require processing steps of this kind and the liquid chemicals that are used are corrosive and heated to high temperatures.
Particulate contaminates are generated in the liquid chemical during the processing of semiconductor wafers or the like and such contaminates may also be present in the initial charge of liquid. Consequently, a continuous flow of liquid is withdrawn from the processing vessel and recirculated back to the vessel through a filter.
My prior copending U.S. patent application Ser. No. 07/434,709 filed Nov. 13, 1989 now U.S. Pat. No. 5,014,737 issued on May 14, 1991, and entitled "Quartz Integrated Trough/Sump Recirculating Filtered High-Purity Chemical Bath", the specification and drawings of which are herein incorporated by reference, discloses a particularly advantageous chemical bath of this type. In the apparatus of that prior application, a sump vessel is situated adjacent one end of the wafer processing vessel and an inclined trough extends along the opposite end and the sides of the processing vessel to deliver overflow from the processing vessel to the sump. A pump draws liquid from the bottom of the sump and recirculates the flow through a filter and into the bottom of the processing vessel. The configuration provides a number of advantages including enabling a very high rate of liquid chemical recirculation while avoiding problems from vortexinq of the flow at the pump intake and from vapor entrainment in the liquid.
In many cases it is desirable that there be a continuing inflow of liquid to the processing vessel from an external source. The external source may, for example, supply an inflow of new liquid chemical to replace liquid losses from such causes as leakage, evaporation or adherence to articles that are withdrawn from the bath. The external source may also be a reprocessor which receives a flow of liquid from the bath and returns purified liquid to the system or may be a source which supplies liquid for still other purposes.
Recirculating chemical baths of this type are adversely affected by variations in the volume of liquid within the bath. Liquid depletion disturbs heat transfer patterns and in extreme cases can cause damaging localized overheating. A build-up of excessive liquid with in the system also has undesirable effects and in extreme cases could result in an overflow. The recirculation system can be a closed loop flow path which inherently returns liquid to the processing vessel at the same rate that liquid is withdrawn from the sump and thus does not in itself have a destabilizing effect. However, inflow of liquid from an external source requires that there be a matching outflow from the bath if liquid volume is to be maintained substantially constant.
The fluid dynamics of a recirculating bath complicate the problem of matching outflow with inflow. Liquid levels in both the processing vessel and the sump undergo pronounced intermittant fluctuations. Immersion of a batch of wafers in the processing vessel causes an abrupt rise of liquid level in the sump. Removal of the batch of wafers causes a temporary lowering of liquid level in the processing vessel followed a slower lowering of liquid level in the sump as the recirculation system returns fluid to the processing vessel. Thus it does not, at first consideration, appear to be feasible to control liquid volume with a liquid level limiting flow outlet at the upper region of the bath. Volume control is further complicated as it is often desirable that the inflow of liquid be adjustable for such purposes as maintaining a desired ratio of flow rates when the recirculation flow is varied by the operator.
Prior arrangements for stabilizing inflow and outflow have resulted in an undesirable complication of chemical bath installations of this general kind. Typically such arrangements are feedback systems which require dynamic valving controlled by flow sensing devices. A less complicated, less costly and more reliable solution would be highly advantageous.
The present invention is directed to overcoming one or more of the problems discussed above.